Oxidation Number of Sulfur in S₂O₈²⁻

Step 1: Recall the rules

1. Oxygen usually has oxidation number -2, except in peroxides (-1).

2. Sum of oxidation numbers = charge of the ion.

Step 2: Identify the type of oxygen

S₂O₈²⁻ has a peroxide bond (O–O):

• 6 normal oxygen atoms → -2 each
• 2 peroxide oxygen atoms → -1 each

Step 3: Set up the equation

Let sulfur = x:

2x + (-12) + (-2) = -2

Step 4: Solve for x

2x - 14 = -2 → 2x = 12 → x = 6

Answer: The oxidation number of sulfur in S₂O₈²⁻ is +6.